1. Field of the Invention
This invention relates to a display device such as a liquid crystal display device which use a TFT (TFT: Thin Film Transistor) as a pixel drive element, and to a method of manufacturing such a device.
2. Description of the Prior Art
FIG. 1 is a cross-sectional view showing a bottom gate thin film transistor structure used as a switching element for a liquid crystal display panel.
A gate electrode 2 of a refractory metal is arranged on the surface of a transparent, insulating substrate 1.
This gate electrode 2 has a taper shape wherein its width expand toward the transparent substrate 1. A silicon oxide film 4 is formed on the transparent substrate 1 on which the gate electrode 2 is arranged via a silicon nitride film 3.
The silicon nitride film 3 prevents infiltration of impurities contained in the transparent substrate into an active layer, described later. The silicon oxide film 4 functions together with the silicon nitride film 3 as a gate insulating film.
A polycrystalline silicon film 5 is formed on the silicon oxide film 4 so as to transverse over the gate electrode 2.
This polycrystalline silicon film 5 is the active layer of the thin film transistor.
A stopper insulating film 6 comprising an insulating material such as silicon oxide is arranged on the polycrystalline silicon film 5. The area of the polycrystalline silicon film 5 covered by this stopper insulating film 6 is a channel area 5c. Other areas of the polycrystalline silicon film 5 are a source area 5s and a drain area 5d.
A silicon oxide film 7 and silicon nitride film 8 are formed on the polycrystalline silicon film 5 on which the stopper insulating film 6 is formed. This silicon oxide film 7 and silicon nitride film 8 protect the polycrystalline silicon film 5 which comprises the source area 5s and drain area 5d.
A contact hole 9 is formed in a predetermined part of the silicon oxide film 7 and silicon nitride film 8 on the source area 5s and drain area 5d. An aluminum electrode 10 connected through this contact hole 9 to the source area 5s and drain area 5d is arranged on the silicon nitride film 8. A planarizing film 11 which is insulating and is transparent to visible light is formed on the silicon nitride film 8 on which the aluminum electrode 10 is arranged. This planarizing film 11 covers imperfections due to the gate electrode 2 or stopper insulating film 6 and makes the surface smooth. A contact hole 12 is formed in the planarizing film 11 on the aluminum electrode 10 on the source area 5s. A transparent electrode 13 made of an ITO (Indium-Tin-Oxide) film connected to the aluminum electrode 10 via this contact hole 12 is arranged so that it extends over the planarizing film 11. This transparent electrode 13 constructs pixel electrode of a liquid crystal display panel.
In this thin film transistor, image data supplied to the aluminum electrode 10 on the drain area is applied to the transparent electrode 13 in response to a scanning control signal applied to the gate electrode 2.
However, the contact resistance of the ITO film was high relative to the aluminum electrode 10, and faulty contacts often occurred. To solve this, there is a technique whereby the contact resistance is lowered by using a high melting metal such as molybdenum (Mo), for example, on the surface of the aluminum electrode 10, but as a molybdenum sputtering step is required after the aluminum sputtering step, there was a disadvantage in that the manufacturing process became complex.